1. Field of the Invention
The present invention relates to fuel injection control systems used for internal combustion engines; these systems are capable of learning at least one parameter indicative of a change in the injection characteristics of a fuel injector.
2. Description of the Related Art
Fuel injection control systems are commonly used to control fuel injection by an injector for each cylinder of an engine.
When a needle valve is rested (seated) upon an inner wall (valve seat) of a fuel injector, the fuel injector is closed. When the needle valve is moved from the valve seat so that a physical distance between the needle valve and the valve seat increases, the fuel injector is opened. This allows a proper amount of pressurized fuel to be metered from a nozzle of the fuel injector.
In fuel injection control using such a fuel injector, fuel-quantity parameters are adjusted depending on a command value indicative of a predetermined amount of fuel so that an actual quantity of pressurized fuel to be squirted out from the fuel injector is controlled. As such fuel-quantity parameters, an optimum fuel injection timing and/or an optimum fuel injection period (valve opening period) can be used.
Even if the fuel-quantity parameters for a plurality of fuel injectors of an internal combustion engine are determined to be equalized to each other, however, individual injection characteristics of the fuel injectors different therebetween may cause variations in actually sprayed fuel quantities therefrom.
In order to address variations in actually sprayed fuel quantities from fuel injectors of an internal combustion engine, a control system is disclosed in U.S. Patent Publication No. 6755176 corresponding to Japanese Unexamined Patent Publication No. 2003-254139.
The control system disclosed in the U.S. Patent Publication works, while performing n split fuel injections by each fuel injector (each cylinder) under idling speed control, to:
measure a value of variations in the engine speed for each cylinder;
compare the measured value for each of the individual cylinders with an average value of all of the measured values for the individual cylinders;
calculate a first fuel quantity correction value for each fuel injector (each cylinder) required to smooth the variations between the respective cylinders based on the comparison result to thereby individually correct an fuel quantity of each fuel injector based on a corresponding one of the firs fuel quantity correction values;
compare an average engine speed with a target engine speed;
calculate a second fuel quantity correction value for all of the fuel injectors required to keep the average engine speed up with the target engine speed based on the comparison result to thereby commonly correct an fuel quantity of each fuel injector based on the second fuel quantity correction value; and
update a previously stored learned value for each fuel injector to a learned value currently obtained by:                dividing the first fuel quantity correction value for each fuel injector by n to obtain a first n-divided correction value therefor;        dividing the second fuel quantity correction value by n to obtain a second n-divided correction value; and        adding, to the previously stored learned value for each fuel injector, the sum of the first n-divided correction value for a corresponding one of the fuel injectors and the second n-divided correction value.        
Specifically, the learned values to be updated for the respective fuel injectors allow the variations in the fuel injection characteristics of each of the fuel injectors to be compensated.
Note that the variations in the fuel injection characteristics of each of the fuel injectors during execution of the pilot fuel injections probably arise depending on variations in the flow passages of pressurized fuel in a corresponding one of the fuel injectors.
Factors responsible for the occurrence of variations in the fuel injection characteristics of each of the fuel injectors however are not limited to the fuel flow-passage variations.
That is, the inventors have found that the factors include variations in the full lift strokes of the needle valves of the individual fuel injectors.
More specifically, the full lift stroke of the needle valve in a fuel injector varies depending on the wearing out of:
the needle valve and the valve seat due to the seating of the needle valve onto the valve seat; and/or
the needle valve and a lift stopper by which the lifting of the needle valve is limited at the full lift stroke.
The more the full lift stroke in a fuel injector increases, the more the fuel quantity to be sprayed therefrom.
The variations in the full lift strokes of the needle valves of a fuel injector have an influence on those in the fuel injection characteristics thereof only when the needle valve is required to move up to its full lift position under high speed and high-load conditions of the internal combustion engine.
In contrast, as described above, the control system disclosed in the U.S. Patent Publication is configured to obtain learned values for each of the fuel injectors for compensating the variations in its fuel injection characteristics only under idling speed control of the internal combustion engine.
This may make it difficult to obtain a learned value for each of the fuel injectors for compensating the variations in its fuel injection characteristics under high speed and high-load conditions of the internal combustion engine.
For this reason, the variations in the fuel injection characteristics of each of the fuel injectors under high speed and high-load conditions of the internal combustion engine may have much influence on the output characteristics of the internal combustion engine. Specifically, the variations in the fuel injection characteristics of each of the fuel injectors under high speed and high-load conditions of the internal combustion engine may cause deterioration of the exhaust characteristic of the internal combustion engine and/or the drivability of a vehicle in which the internal combustion engine is installed.
Thus, it is important to learn, under high speed and high-load conditions of the internal combustion engine, the amount of deviation from the fuel injection characteristics of each of the fuel injectors in order to keep the output characteristics of the internal combustion engine in good conditions.